FIG. 1 illustrates a basic three phase inverter 10. In general, the three phase inverter 10 includes a voltage regulator and isolation circuit 12 connected to the power lines of the power grid and providing isolation from the power grid. Optionally, the voltage regulator and isolation circuit 12 may provide voltage reduction and/or voltage regulation. The voltage regulator 12 generates a direct current (DC) positive supply voltage (+BUS) that is referenced to earth ground (GND). Three “legs” of the three phase inverter 10 are connected between the positive supply voltage (+BUS) and ground (GND). Each of the legs operates to provide one of the three motor currents (IM1, IM2, IM3). Transistors 14 and 16 from the first leg, transistors 18 and 20 form the second leg, and transistors 22 and 24 form the third leg. Each of the transistors 14–24 are controlled by a switching driver 26 such that the three phase inverter 10 generates the three motor currents (IM1, IM2, IM3) that are used to drive a three phase motor.
In order to properly control a three phase motor, it is desirable to sense, or monitor, one or more of the motor currents (IM1, IM2, IM3). In reality, once two of the three motors currents are known, the third motor current can be determined based on Kirchoff's current law which states that the sum of all current flowing into a node is zero, as is well known in the art. Sensing the motor currents with resistors is typically done in one of four ways. A first method places a single resistor between transistors 16, 20, and 24 and ground (GND), thereby sensing one or more of the motor currents. Although simple, this method does not provide enough information to determine each of the motor currents, and is therefore not suitable for high performance motor control.
A second method inserts a first sense resistors between one of the transistors 16, 20, 24 and ground (GND) and a second sense resistor between another of the transistors 16, 20, 24 and ground (GND). However, the motor current only flows in the sense resistors when the respective transistor 16, 20, or 24 is on. Therefore, sampling or estimation techniques must be used, which increases complexity and noise.
A third method inserts a first sense resistor in series with one of the three motor phases and a second sense resistor in series with a second of the three motor phases, such that one of the three motor currents (IM1, IM2, IM3) follows directly through each of the sense resistors. Although this method provides good dynamic control of the motor, a small differential voltage from the sense resistors must be separated from a large AC common mode voltage due to the switching of the transistors 14–24. Circuits capable of rejecting the large AC common mode voltage are relatively expensive and/or large, and are thus less suited for small volume, high performance, motor drives.
A fourth method uses two upper and two lower sense resistors. For example, a first upper sense resistor is placed between transistor 14 and the positive supply voltage (+BUS), and a corresponding first lower sense resistor is placed between the transistor 16 and ground (GND). A second upper sense resistor is placed between the transistor 18 and the positive supply voltage (+BUS), and a corresponding second lower sense resistor is placed between the transistor 20 and ground (GND). The upper sense resistors are used to sense the motor currents (IM1, IM2) when respectively transistors 14 and 18 are active, and the lower sense resistors are used to sense the motor currents (IM1, IM2) when respectively transistors 16 and 20 are active. In principal, the sum of the currents through the upper and lower sense resistors in each leg is a continuous output current corresponding to the motor currents. In practice, outputs of the upper sense resistors must be amplified and differentially shifted down to ground where the common mode is rejected and the output of the lower sense resistors are added. Commonly used level shifters are a current mirror or a common base amplifier built with high voltage transistors. These transistors are temperature sensitive. For example, the gain and base to emitter voltage of the transistors varies with temperature. Thus, an offset is introduced into the output signal that varies with temperature. Further, the offset can be a substantial fraction of the maximum sensed signal.
Thus, there remains a need for a system for sensing one or more motors currents from a three phase inverter that avoids the problems of the previous systems, including those described above.